As impact crushing becomes more prevalent in the production of friable as well as some harder types of aggregates, it is natural for those not completely familiar with the process, or recent improvements in technology, to wonder why impactors are being specified in situations previously often considered uneconomical or, at best, marginally feasible for this type of equipment. The simple answer is that the rules for determining the economic feasibility of utilizing impact systems have changed over the last few years, and are continuing to change at an accelerating pace.
Impact Crusher Advantages
Before discussing the new economics of impact crusher usage, it might be well to briefly state the original, and still valid, advantages of the impact crusher in the production of friable aggregates. Low original cost has always been the primary advantage of this equipment. For a given application this first cost may well be only a quarter as much as for compression type crusher installation. Second, a greater reduction ratio can be achieved. Many installations are now producing specification stone from run-of-quarry feed in a single pass through a primary impact unit. Third, impact crushers are capable of accepting larger feed sizes, with units now in operation handling feed sizes in excess of 65″ x 65″ x random length. This larger feed size capability reduces material hang-up in the feed hopper and the need for use of either secondary blasting or a headache ball to reduce oversize rock in the quarry prior to transporting it to the primary crusher. And fourth, impact crushing, by its very nature, produces a higher quality product. It produces a dense, cubical product without the slivers or slabs prevalent in other methods, making it easier for plant operators to meet the stringent specifications laid down by many customers.
Each of these inherent advantages leads automatically to certain economic advantages, as well as to some recognized disadvantages. However, some of the potential economies have not, until recently, been fully exploited. Nor is it well known among all producers how some of the old disadvantages have been overcome in some of the newer units and systems now appearing in newer quarries.
Briefly, compression is the forcing of two surfaces toward one another to crush the material caught between them. Impact crushing can be of two variations—gravity and dynamic. An example of gravity impact would be dropping a rock onto a steel plate.
Crusher Technology
Design improvements in many of the impactors being manufactured today have a great deal to do with changing earlier ideas concerning the economics of this type of equipment vs. nominal installations. But, of equal importance, is the systems design approach now being offered by the leading manufacturers to solve aggregate production problems. In any technology, when a thorough systems design approach is undertaken in an effort to better utilize the inherent advantages of a specific product, the results may be much more rewarding than mere use of the product advantages by themselves. This is proving true in aggregate production as more quarry operators are calling in impactor manufacturers during the early planning stages and letting them design complete production systems that utilize all the advantages of today’s crusher technology.
The flow diagram at the left is from one manufacturer’s brochure. He uses it to indicate not only that he welcomes the opportunity to custom engineer a complete system for his customers, but also to show through call-outs, which have been deleted for this article, that his company actually designs and manufactures about ninety percent of the machines, parts, assemblies and other items necessary for this type of total plant.
This willingness by the leading crusher manufacturers to assume total responsibility for turnkey installations has forced technological improvements that might still be years away if manufacturers still sold, and the industry still purchased, only bits here and pieces there. One leading manufacturer readily admits that improvements he has made in his crushers have forced changes in his lines of material handling equipment; that changes in quarrying techniques and demands for increased capacity have speeded improvements in his crusher design, screening systems, conveyor parts and systems, feeder arrangements, and drying and storage equipment; that sometimes these domino-type changes go full circle and start yet another round of changes and improvements, all of benefit to the system buyer, and many of which might never have been made were they not caused by total systems development responsibility.
Along with these “chain effect” improvements in complete aggregate processing systems are the steady and, in some cases, almost dramatic improvements made in impact crushers themselves. Considerable R&D work by leading manufacturers has led to larger, more versatile, more durable, and more efficient impact crushers in recent years that are much easier and more economical to maintain.
Crushing equipment, whether primary or secondary in nature, represents one of the most important aspects in the overall operation of minerals* processing. In an area which includes the vast range of ores, minerals, coals, stone and rocks, etc., a processing plant’s crusher selection must be given special consideration. Performance, cost, availability and versatility are the prime goals.
Crusher Selection
For the crusher to perform its role in the processing cycle with maximum efficiency and economy it must be matched to the task. Determining factors in crusher selection break down into four categories: material to be crushed; feed size; product size; and expected capacity. The right crusher should also have the lowest power requirements per ton of finished product, and operate with minimum maintenance and downtime.
There are a variety of crushers to meet the needs of today’s industrial requirements. Exactly how is a determination made on the proper crusher? Extensive study and evaluation of the above facts concerning material, feed size, etc. will be an essential guideline. Next to be considered is the actual mechanical method of crushing to be used. Generally, crushing methods include compression, impaction, attrition, and shearing.
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